PENTACRINOID LARVAE 217 



The measurements of the specimens and the number of columnals in the stem and of 

 brachials in the arms are as follows : 



10. Crown 2-9 mm.; stem: 39 columnals, 9-6 mm.; arms of about 8 brachials. 



11. Crown 2-5 mm.; stem: 43 columnals, 10 mm.; arms of about 8 brachials. 



12. Crown 2-5 mm.; stem: 34 columnals, 11 mm.; arms of 8-10 brachials. 



13. Crown 2-9 mm.; stem: 39 columnals, 10 mm.; arms of 8-10 brachials. 



14. Crown 2-7 mm.; stem: 44 columnals, 10-4 mm.; arms of 10 brachials. 



The next specimen is considerably older than any of the preceding : 



15. Length of crown 4 mm. (Fig. 24 b-e); length of column 10-5 mm. 



The stem is of 39 columnals and a thick slightly lobed terminal plate. The first 

 columnal is incomplete so that the second, although for the most part in touch with it, 

 is in touch with the basal cup too; it is not shown in the figure, in which the most 

 proximal columnal is the second. The second to seventh columnals are short. The third 

 is narrower than the second. The fourth to seventh are of the same diameter as one 

 another, narrower than the third. There is a gradual increase in length from the fourth 

 to the seventh columnal, the seventh being half as long as wide. The tenth is as long as 

 broad; the eleventh to the fourteenth are slightly longer than broad (Fig. 24 c). The 

 remaining columnals gradually decrease in length : the most distal are wider than long 

 (Fig. 24 d). Only faint traces of the encircling girdles of the columnals remain. The 

 articulating surfaces of the two ends of each columnal, or at least of the longer columnals, 

 are broadly oval with the longer axis of one end at right angles to that of the other ; this 

 is only just perceptible. 



The sides of the basi-radial cup are nearly straight ; its height is about half its distal 

 diameter. The basals are considerably longer than the radials. The radials, except for 

 the posterior pair, are in broad contact with one another; beyond the parts of the 

 lateral edges which are in contact they narrow a little. The middle of the wide distal 

 edge is indented for the articulation of the costal ; it occupies about a half of the distal 

 edge of the radial. The lateral edges of the posterior radials meet proximally, but are 

 cut away beyond to allow room for the radianal plate: it follows that both the radials 

 are asymmetrical, and they are equally so. 



The radianal plate is diamond-shaped and longer than broad. Its proximal half is in 

 contact with the radials and costals ; the distal half lies on the anal tube which is much 

 wider than it and has gently converging lateral edges and a straight distal margin. It is 

 very near the edge of the disk : the level of the radianal plate is only a little below that 

 of the radials. 



The narrow end of the posterior oral may be seen beyond the anal tube, far below its 

 level, curving over the disk (Fig. 24 b). The other oral plates bend sharply in over the disk 

 and rapidly narrow in their distal halves ; they are separated from the radials by a narrow 

 naked strip of perisome. The surface of the wider proximal part of each oral is pitted 

 and some of the pits carry glandular sacs. 



The arms are of 15-16 brachials with large side- and long cover-plates (Fig. 24 e) and 

 irregularly arranged sacculi. On the distal portions of some arms there is one sacculus 



D .Will '3 



